The present invention relates to a surface elastic wave device, and more particularly to a surface elastic wave device having input and output electrodes disposed on a surface layer of a piezoelectric body for sending and receiving a surface elastic wave between the electrodes to produce a signal having desired frequency characteristics from a supplied high-frequency signal.
Surface elastic wave devices utilize the properties of a surface elastic wave such that most of its energy is propagated along a surface of a solid body. Such surface elastic wave devices are widely used in oscillator circuits, filter circuits, delay circuits, and the like in various pieces of communication or electronic equipment. The surface elastic wave device comprises, for example, a piezoelectric crystal body with a ground surface and input and output electrodes in form the of a pair of thin metallic films on the surface of the piezoelectric crystal body for sending and receiving a surface elastic wave between the electrodes. More specifically, when the input electrode on the piezoelectric crystal body is supplied with a high-frequency signal, the piezoelectric body vibrates due to the piezoelectric effect, and the vibration is transmitted from the input electrode and propagated as an elastic wave along the surface of the piezoelectric body toward the output electrode. The output electrode receives the elastic wave and issues a high-frequency output signal which is produced by the reverse piezoelectric effect of the piezoelectric body.
Recently, surface elastic wave devices have been finding wide use as filters since the phase or amplitude characteristics thereof can be selected as desired.
FIG. 1 of the accompanying drawings illustrates a conventional surface elastic wave filter of a basic configuration. The surface elastic wave filter has a piezoelectric substrate 12 and a pair of input and output electrodes 16, 18 disposed on the piezoelectric substrate 12 in spaced-apart relationship. The input and output electrodes 16, 18 have common electrodes 16a, 16b and 18a, 18b, respectively. The input electrode 16 includes a plurality of parallel interdigitating electrode fingers 20a, 20b, 20c, 20d, 20e extending between the common electrodes 16a, 16b. The output electrode 18 includes a plurality of parallel interdigitating electrode fingers 24a, 24b, 24c, 24d, 24e extending between the common electrodes 18a, 18b.
A high-frequency signal S.sub.1 is applied to the input electrode 16 between the common electrodes 16a, 16b, and a high-frequency signal S.sub.2 having desired frequency characteristics is produced from the output electrode 18 between the common electrodes 18a, 18b.
While the basic configuration of the surface elastic wave filter is shown in FIG. 1, the interdigitating electrode fingers may be modified in shape in an actual application to obtain desired output signal frequency characteristics. For example, the interdigitating electrode fingers may be shaped to widen the passband of the filter and eliminate ripples from the frequency characteristics in the passband.
Another conventional surface elastic wave filter is illustrated in FIG. 2 of the accompanying drawings. The surface elastic wave filter comprises a piezoelectric substrate 23 and a pair of input and output electrodes 36, 38 disposed on the piezoelectric substrate 23 in spaced-apart relationship. The input and output electrodes 36, 38 have common electrodes 36a, 36b and 38a, 38b, respectively. The input electrode 36 includes divergent electrode fingers 40a, 40b, 40c positioned between the common electrodes 36a, 36b and electrode fingers 40d, 40e disposed in spaces defined between the electrode fingers 40a, 40b, 40c in interdigitating relationship. Similarly, the output electrode 38 also includes divergent electrode fingers 44a, 44b, 44c and electrode fingers 44d, 44e disposed between the common electrodes 38a, 38b in interdigitating relationship. The pitch or distance, in the direction x in which the surface elastic wave is propagated, between adjacent two of the electrode fingers from each of the common electrodes 36b, 38b progressively varies from a minimum value of P.sub.L to a maximum value of P.sub.H across propagation paths a through n which are juxtaposed in a direction y normal to the direction x of propagation on the piezoelectric substrate 32.
The common electrodes 36a, 36b are supplied with a high-frequency signal S.sub.4 therebetween, and a high-frequency signal S.sub.5 having desired frequency characteristics is picked up from between the common electrodes 38a, 38b.
It may be considered that the interdigitating electrodes of the shape shown in FIG. 2 provide a number of parallel-connected surface elastic filters having different interelectrode pitches in respective small segments .DELTA.y in the direction y.
As is well known in the art, the frequencies that can be passed through a surface elastic wave filter are determined by the interelectrode pitch. Assuming that the speed of propagation of a surface elastic wave through the filter of FIG. 2 is indicated by v, the frequencies that can be passed through the filter range from: EQU f.sub.L =P.sub.L /v . . . (1)
to EQU f.sub.H =P.sub.H /v . . . (2)
In this frequency range, the pitch P(y) in the direction y normal to the direction x of propagation of the surface elastic wave smoothly varies. The frequency range is wide with no ripples therein. In the arrangement of FIG. 2, the interelectrode pitch is varied in each of the input and output electrodes 36, 38. However, the interelectrode pitch of the interdigitating electrode fingers in either one of the input and output electrodes 36, 38 may be varied. FIG. 3 shows phases and amplitudes (insertion losses) at respective frequencies of the surface elastic wave filter of FIG. 2, and FIG. 4 illustrates phases and insertion losses at respective frequencies of the surface elastic wave filter of FIG. 2.
The surface elastic wave filter shown in FIG. 2 is capable of providing linear-phase frequency characteristics in a relatively wide range, but fails to achieve nonlinearphase frequency characteristics.
More specifically, a signal which has passed through an LC filter suffers overshooting or undershooting due to the transient of the filter, and hence the signal from the filter has a distorted leading or trailing edge, i.e., a characteristic deterioration such as curved group delay characteristics is developed. To avoid this problem, a signal produced from an LC filter is passed through a surface elastic wave filter in many instances, so that desired amplitude and phase characteristics will be given to the signal by the surface elastic wave filter. Where such a surface elastic wave filter is employed, more accurate characteristics are desired, and ripples produced by the surface elastic wave filter itself may cause a problem. There is therefore a demand for a surface elastic wave filter which can provide nonlinear-phase frequency characteristics with low ripples and has a wide frequency range.